1. Field of the Invention
The present invention relates to a method and an apparatus for printing an image on an advancing recording medium while adjusting printing position of the image to position of the advancing recording medium relative to a printing head.
2. Background Arts
There have been various kinds of line printers which record images on advancing recording medium by using a linear printing head that extends in a main scan direction transverse to the advancing direction of the color thermosensitive recording medium. In thermal type line printers, the printing head is provided with an array of heating elements. In optical type line printers, the printing head is provided with an array of light emitting elements.
In either type, it is required to position the individual images properly on the recording medium. That is, the image should be printed with predetermined margins or with no margin on the recording medium. By controlling position to start and stop printing the image on the recording medium or positions to cut the recording medium after the printing, it is possible to omit end margins before and behind the image in the paper advancing direction, or to provide the end margins with predetermined widths.
As for lateral sides of the image in the paper advancing direction, margins may be omitted by setting a maximum length of a main scanning line provided by the linear printing head to be equal to the width of the recording medium. By adjusting the length of the main scanning line, it is also possible to provide side margins of predetermined widths on the opposite lateral sides of the image. However, for many reasons, the advancing recording medium can be shifted in the main scan direction relative to the printing head or the main scanning line thereof. The recording medium can also inclines relative to a sub scan direction perpendicular to the main scan direction. Then the side margins would not have the predetermined widths, or the printed image would be inclined to the side edges of the recording medium. Where the image is intended to be printed without margins, undesirable blanks would be provided on the periphery of the image, or the printed image lacks some marginal portion. Therefore, it has been necessary to correct the position of the recording medium in the main scan direction frequently by adjusting paper guide members or the like.
A thermal printer disclosed in JPA 10-44482 suggests using a thermal head with a heating element array whose length is longer than a maximum width of the color thermosensitive recording paper. By shifting those heating elements which are used for recording among the array of the heating elements, the line provided by the thermal head is adjusted in length and position in the main scan direction to the width of the recording paper and the relative position of the recording paper to the thermal head. To print an image without the side margins, this prior art also suggest adding pixel data to original image data, thereby to print additional pixels on the fringe of the original image in accordance with the shifted amount of the color thermosensitive recording paper in the main scan direction. The additional pixels may have the same graduation value as the adjacent marginal pixel of the original image, or decreasing graduation values.
However, the thermal printer of this prior art does not have a device for detecting the shift amount of the color thermosensitive recording paper in the main scan direction. Therefore, it is necessary to preliminary print a test image in order to check the position of the test image as well as the marginal conditions, and then adjust paper guide members so as to optimize the marginal conditions. Especially when the advancing recording paper is not perpendicular to the main scan direction, it is impossible to eliminate undesirable blanks perfectly.
Furthermore, in the second method of the prior art, some of those heating elements which are driven with the original image data can be displaced from the color thermosensitive recording paper. In that case, the printed image would lack some marginal part of the original, and also heat energy is not transmitted from these heating elements to the ink sheet or the color thermosensitive recording paper, so the heat energy is accumulated and over-heats these heating elements.
JPA 6-32009 discloses a serial printer, wherein a photoelectric sensor is mounted to a serial printing head that scans the recording paper in the main scan direction. Before starting printing, the serial printing head moves in the main scan direction, so the photoelectric sensor detects opposite side edges of the recording paper. Thus, image printing position by the serial printing head is adjusted to the positions of the side edges in the main scan direction.
According to this prior method, it is easy to print an image without any margins even when the recording paper is shifted in the main scan direction. However, since the position of the recording paper in the main scan direction is detected once prior to starting printing, if the recording paper is not advanced in the perpendicular direction to the main scan direction during the printing, the printed image would be inclined to the side edges of the recording paper.